So i've got this model of a car I want to make, it's a model of an actual solar powered car we're building at UNSW. However whenever I try to upload the model, it comes back with an error message and I really don't know why. Note we've already used the file to build the actual car, from moulds, so I don't think it's because we've made a dodgy model.

Three problems.
1) Your model has 17 shells and 582 holes. You should close all the holes and really should convert it to a single shell.
2) The righthand front wheel is duplicated in the straight plus right-hand and left-hand turn positions.
3) No printer Shapeways has is large enough to print your actual-sized car. <GRIN>

Your model is in real-world millimeters. You need to scale it down to something that will actually fit in the printer. (Shapeways doesn't have an option for rescaling the model AFTER uploading).

I converted your file to an STL for you.
Grab yourself a copy of the free Netfabb Basic from www.netfabb.com and load this file up.

1) Use the Netfabb tools to resize the model to some reasonable size. I would suggest somewhere between 2 and 4 inches long, depending on where you want to end up with cost.
2) Click on Repair, then "Close Holes", then "Remove Degenerate Faces"=0.001mm
3) Export the STL

4) Then, upload the model to cloud.netfabb.com in order to get all the shells welded together.

I'm looking forward to the day when we can order a 3D printed full size car!

I'm going to say the technology will be available within 5 years to print an entire car in situ. All we need really is a breakthrough in multi-material capability such as metals in conjunction with polymers.